This protocol will examine whether oral tolerance, which is a state of immunologic hyporesponsiveness following the ingestion of an antigen, exists in man. A purified protein antigen that is not a component of the normal human diet, namely keyhole limpet hemacyanin (KLH), will be fed to human volunteers. Samples of peripheral blood and secretions will be obtained after feeding to determine whether any mucosal immunization takes place. Volunteers will then be immunized parenterally with KLH and their humoral and cellular responses measured and compared to volunteers not fed but simply immunized parenterally. A second aim of the project is to determine whether co-ingestion of a mixture of KLH and cholera toxin B subunit alters the immune response to KLH. Assuming that oral tolerance is demonstrated, the mechanism of that tolerance will be explored to see specifically whether the tolerance involves B cells, T cells or both. The information generated in this study will be relevant to the understanding of adverse reactions to food allergy, autoimmunity, and inflammatory bowel disease.